JP2005173378A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which enables to suppress voids in halftone image parts adjacent to solid images regardless of a write system or a ratio of coloring components in toner. <P>SOLUTION: In the image forming apparatus adopting a two-component developing system, TC×Vpp/(M/A)≤14 is satisfied, wherein Vpp is a peak-to-peak voltage value [kV] of an AC voltage for applying an AC developing bias to a developing sleeve 16 and TC is a toner weight concentration [wt%] and (M/A) is the amount of sticking per unit area for obtaining a maximum image density, and the shortest distance Gp between the developing sleeve and a photoreceptor is set to 0.3 to 0.6 [mm], and a developer quantity Dq per unit area on the developing sleeve is set to 40 to 70 [mg/cm<SP>2</SP>], and a linear speed ratio Vs/Vp of the surface of the developing sleeve to the photoreceptor 1 is set to 1.2 to 2.0, and a frequency f of the AC developing bias applied to the developing sleeve is set to 2.0 to 8.0 [kHz]. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine of these. More specifically, the present invention relates to an image forming apparatus equipped with a two-component developing type developing device that forms an image using a two-component developer composed of a carrier and a toner.

  In general, in an electrophotographic type or electrostatic recording type image forming apparatus, an electrostatic latent image corresponding to image information is formed on a latent image carrier such as a photosensitive member or a photosensitive belt, and the electrostatic latent image is formed. On the other hand, development is performed by a developing device to obtain a visible image. In recent years, two-component developers composed of toner and carrier (hereinafter simply referred to as “development”) from the viewpoint of transferability, halftone reproducibility, stability of development characteristics with respect to temperature and humidity, and the like. It is the mainstream to use a two-component development method using an “agent”.

  An example of a developing apparatus that uses such a two-component developing system is shown in FIG. As shown in the figure, the developing device is provided with a developing sleeve 41 having a developer carrier disposed so as to be partially exposed from the opening of the casing. In addition, a developer reservoir 45 that contains a two-component developer, a first transport screw 42 that is a developer supply member provided in the developer reservoir 45, a second transport screw 43, and developer regulation A doctor blade 44 as a member is provided. The developing sleeve 41 includes a sleeve formed in a cylindrical shape and a magnet body (magnet roller) fixedly disposed inside the sleeve.

When the developing device 40 enters the developing operation, the first transport screw 42 and the second transport screw 43 rotate in the reverse direction to circulate and transport the developer in the developer reservoir. At this time, the toner in the developer is frictionally charged to such a polarity that it adheres to the electrostatic latent image by stirring with the carrier. The developer conveyed to the first conveying screw 42 is pumped onto the developing sleeve 41 by the magnetic field on the developing sleeve. Then, the carrier to which the charged toner is attached is spiked in a brush shape so as to follow the lines of magnetic force formed on the surface of the developing sleeve. These developers are conveyed along with the developing sleeve by the magnetic field on the developing sleeve, and the layer thickness thereof is regulated by the doctor blade 44. Then, the developer that has passed through the doctor blade 44 is transported to a developing area facing the photoreceptor 1. The brush-like developer thus transported to the developing area comes into contact with the surface of the photosensitive member so as to bend as the surface of the developing sleeve moves, and supplies toner to the electrostatic latent image.
The toner image formed on the photoreceptor is finally transferred to a recording medium such as paper, and a fixed image is formed through a process such as heating.
The developing sleeve surface moving speed is set to be faster than the photosensitive member surface moving speed. If the difference between the developing sleeve surface moving speed and the photosensitive member surface moving speed is eliminated, or if the photosensitive member surface moving speed is set faster than the developing sleeve surface moving speed, the amount of developer supplied to the developing region is reduced and sufficient. This is because a density image may not be obtained.

  However, the fixed image obtained in this way has a problem that white spots occur in the halftone image portion adjacent to the solid image. The white spots are white spots on the entire circumference of the halftone image adjacent to the solid image (hereinafter referred to as “all-round white spots”), and white spots on the halftone image adjacent to the leading edge of the solid image (hereinafter referred to as “white spots”). , "The tip is blank"). The mechanism by which these white spots occur will be described later.

In order to solve the above-described problem of white spots on the tip, Patent Document 1 proposes a configuration in which the thickness of the toner layer in the solid region is about 1 to 1.2 layers. In addition to this, the developer adhesion amount per unit area on the developing sleeve is M [g / m ² ], the toner weight concentration TC in the developer is Rt [wt%], and the surface of the developing sleeve with respect to the photosensitive member When the speed ratio is v, the toner transfer rate from the developing sleeve to the photosensitive member is s [wt%], and the transfer rate in the transfer process is k [wt%], the following Equation 1 is satisfied. A method has been proposed.
<Equation 1> M · Rt · v ≧ a / (k · s × 10 ⁻⁶ )
By these, white spots in the halftone image portion adjacent to the solid image can be improved.
JP-A-10-186842

By the way, it is known that the problem of white spots in a halftone image portion adjacent to a solid image can be improved to some extent by using a so-called binary writing method in which image data is digitally processed and exposure light is irradiated onto a photosensitive member. It has been. However, there are still many problems in the case of the multi-value writing method, and improvement of this is desired. The reason will be described below.
FIGS. 1A and 1B are diagrams for explaining an image forming method in binary writing and multi-value writing, respectively. An area indicated by M in the figure is a solid image part, and an area indicated by K in the figure is a halftone image part. In the binary writing method, as shown in FIG. 1A, a halftone image appears according to the density of dots. On the other hand, in the multi-value writing method, as shown in FIG. 1B, the halftone image is displayed by adjusting the toner adhesion height.
A dotted line N shown in the halftone image portion K in FIGS. 1A and 1B indicates the toner adhesion amount when white spots occur. Comparing the dotted line N in FIGS. 1A and 1B, the multi-value writing method in FIG. 1B has a greater toner adhesion amount than the binary writing method in FIG. It can be seen that the amount of decrease is large. Therefore, the problem of white spots is more serious in the multi-value writing method than in the binary writing method.

  On the other hand, resource saving has become an important issue in recent years. Therefore, it is desired to reduce the amount of toner used while ensuring a desired image density. In order to realize this, a technique of increasing the ratio of the coloring component among the toner components is used. However, when an image density is obtained using a high color toner, the problem of white spots becomes more serious. This is because the toner adhesion amount for forming an image can be reduced as compared with the conventional one, and therefore, when the above white spot phenomenon occurs, it appears more prominently.

  In the above-mentioned Patent Document 1, there is no description as to whether or not white spots in a halftone image portion adjacent to a solid image portion can be improved even with a multi-level writing method and toner with a high degree of coloring.

  The present invention has been made in view of the above background, and its purpose is to suppress white spots in a halftone image portion adjacent to a solid image regardless of the writing method and the ratio of the coloring component in the toner component. An image forming apparatus capable of doing this is provided.

In order to achieve the above object, the invention of claim 1 is a developer carrying member that carries a two-component developer composed of a toner and a carrier, and development that applies a developing bias containing an AC component to the developer carrier. In an image forming apparatus comprising a developing device having a bias applying unit and an image carrier that forms an electrostatic latent image facing the developer carrier, the following conditions (1) to (5) are satisfied: It is characterized by doing.
Condition (1): TC × Vpp / (M / A) ≦ 14 (where Vpp is a peak-to-peak voltage value [unit: kV] of an AC voltage for applying an AC developing bias to the developer carrier. TC is the toner weight concentration [unit: wt%], and (M / A) is the adhesion amount [unit: mg / cm ² ] per unit area for obtaining the maximum image density.)
Condition (2): The closest distance Gp between the developer carrier and the image carrier is 0.30 [mm] or more and 0.60 [mm] or less. Condition (3): Unit on the developer carrier Developer amount Dq per area is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less Condition (4): The linear velocity ratio Vs / Vp of the surface of the developer carrier to the image carrier is 1.2 or more and 2.0 or less Condition (5): The frequency f of the AC developing bias applied to the developer carrying member is 2.0 [kHz] or more and 8.0 [kHz] or less. The image forming apparatus according to claim 1, wherein a peak-to-peak voltage value Vpp of an AC voltage for applying an AC developing bias to the developer carrying member is 0.4 [kV] or more and 0.8 [kV]. It is characterized by the following.
According to a third aspect of the present invention, in the image forming apparatus of the first or second aspect, the toner weight concentration TC is 4 [wt%] or more and 8 [wt%] or less.
According to a fourth aspect of the present invention, there is provided a developer carrier that carries a two-component developer composed of a toner and a carrier, and a development bias application unit that applies a development bias containing an AC component to the developer carrier. In an image forming apparatus including a developing device and an image carrier that forms an electrostatic latent image facing the developer carrier, the following conditions (6) to (10) are satisfied: Is.
Condition (6): Gp × Dq / (M / A) ≦ 60 (where Gp is the closest distance [unit: mm] between the developer carrier and the image carrier, and Dq is the developer carrier) The amount of developer per unit area [unit: mg / cm ² ].)
Condition (7): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (8): Peak-to-peak voltage value of AC voltage for applying AC developing bias to the developer carrying member Vpp is 0.4 [kV] or more and 1.0 [kV] or less. Condition (9): The linear velocity ratio Vs / Vp of the surface of the developer carrier to the image carrier is 1.2 or more and 2.0. The following condition (10): The frequency f of the AC developing bias applied to the developer carrying member is 2.0 [kHz] or more and 8.0 [kHz] or less. In the forming apparatus, the closest distance Gp between the developer carrier and the image carrier is 0.25 [mm] or more and 0.45 [mm] or less.
According to a sixth aspect of the present invention, in the image forming apparatus of the fourth or fifth aspect, the developer amount Dq per unit area on the developer carrying member is 30 [mg / cm ² ] or more and 60 [mg / cm ² ] It is characterized by the following.
The invention of claim 7 further comprises a developer carrying member carrying a two-component developer comprising toner and carrier, and a developing bias applying means for applying a developing bias containing an AC component to the developer carrying member. In an image forming apparatus including a developing device and an image carrier that forms an electrostatic latent image facing the developer carrier, the following conditions (11) to (15) are satisfied: Is.
Condition (11): (Vs / Vp) / f / (M / A) ≦ 0.75 (where Vs / Vp is the linear velocity ratio of the developer carrier surface to the image carrier surface, and f is (It is the frequency [unit: kHz] of the AC developing bias applied to the developer carrying member.)
Condition (12): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (13): Peak-to-peak voltage value of AC voltage for applying AC developing bias to the developer carrying member Vpp is 0.4 [kV] or more and 1.0 [kV] or less Condition (14): The closest distance Gp between the developer carrier and the image carrier is 0.30 [mm] or more and 0.60 [mm] The following condition (15): the developer amount Dq per unit area on the developer carrying member is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less. In the image forming apparatus of No. 7, the value of the linear velocity ratio Vs / Vp of the developer carrying member surface to the image carrying member surface is set to 1.1 or more and 1.6 or less.
According to a ninth aspect of the present invention, in the image forming apparatus according to the seventh or eighth aspect, the frequency f of the AC developing bias applied to the developer carrying member applied to the developer carrying member is 4.0 [kHz] or more. 12.0 [kHz] or less.
According to a tenth aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, fifth, sixth, seventh, eighth, or ninth aspect, an electrostatic latent image is formed on the image carrier. A multi-value writing method is adopted as the method.
According to an eleventh aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, or tenth aspect, toner adhesion per unit area for obtaining the maximum image density is achieved. The amount (M / A) is 0.40 [mg / cm ² ] or less.

According to these inventions, there is an excellent effect that white spots in the halftone image portion adjacent to the solid image can be suppressed regardless of the writing method and the ratio of the coloring component in the toner component.
The reason why the above effect is obtained will be described below.

First, the mechanism by which white spots occur in the halftone image portion adjacent to the solid image will be described.
FIG. 2A is a diagram for explaining a mechanism in which “all-round white spot” occurs in a two-component developing type developing apparatus. This figure shows a developing region where the photosensitive member 1 as an image carrier and the developing sleeve 16 as a developer carrier face each other. In the drawing, M, K, and J represent a latent image portion for forming a solid image, a latent image portion for forming a halftone image, and a non-latent image portion, respectively. Arrows p and q in the figure indicate the direction of the electric field. As shown in the drawing, on the surface of the developing sleeve facing the boundary region between the latent image portion M for forming a solid image and the latent image portion K for forming a halftone image, an arrow q in the figure is shown due to an edge effect. As shown in FIG.
FIG. 3 is a diagram for explaining the potential pattern at this time. In the drawing, Va, Vb, Vc, and Vd represent the surface potential of the solid image portion, the surface potential of the halftone image portion, the potential of the developing sleeve, and the potential of the non-latent image portion, respectively.
Due to the edge effect, a potential fluctuation as shown in the figure occurs in the boundary region between the latent image portion M for forming the solid image portion and the latent image portion K for forming the halftone image portion. The toner adhesion amount on the fixed image varies with the potential variation. That is, the toner adhesion amount increases at the solid image edge portion, and the toner adhesion amount decreases at the halftone image edge portion. At this time, even if the toner adhesion amount increases at the solid image edge portion, since the image is originally a solid image, a large difference is not visually observed on the fixed image. On the other hand, at the edge portion of the halftone image, a reduction in the toner adhesion amount appears clearly in the fixed image. Therefore, a halftone blank in a solid image adjacent portion becomes a problem.

  FIGS. 4A to 4C are views for explaining a mechanism that causes “blank tip” in a two-component developing type developing apparatus. FIGS. 4A to 4C show, in order, the movement of the tip of a brush-like developer (magnetic brush) that gradually approaches the surface of the photoreceptor 1 as a latent image carrier. doing. The brush moves in the direction indicated by the arrow L in the figure. In the figure, M and K indicate a latent image portion for forming a solid image and a latent image portion for forming a halftone image, respectively. 4B shows the boundary between the latent image portion M that forms a solid image and the latent image portion K that forms a halftone image in the developing region where the photosensitive member 1 and the developing sleeve 16 face each other. FIG. 4A and FIG. 4C show the states before and after that. In FIG. 4A, a halftone image just developed is formed downstream of the magnetic brush indicated by the solid line of the photosensitive member 1 in the rotation direction in the drawing. Here, the photoconductor 1 actually rotates in the direction of the arrow r in the figure, but since the developing sleeve 16 usually moves faster than the photoconductor 1, FIGS. 4A to 4C. In FIG. 2, the photosensitive member 1 is illustrated as being stationary.

  As shown in FIGS. 4B and 4C, the magnetic brush that rubs the latent image portion K that forms the halftone image after entering the development area has a solid image portion as shown in FIG. The toner has been supplied to the latent image portion M to be formed. For this reason, when the toner consumption proceeds during the period facing the latent image portion M for forming the solid image portion and the latent image portion K for forming the halftone is rubbed, FIG. As shown, toner is depleted on the surface of the carrier 35 at the tip of the magnetic brush. At this time, the tip of the magnetic brush is in a state where charges (counter charger) having a polarity opposite to that of the toner are generated. Such a magnetic brush electrostatically attracts toner attached to one end of the rear end portion of the latent image portion K for forming a halftone image as shown in FIG. As a result, even if the toner 36 is once supplied by the magnetic brush in the developing area, it is pulled back to the surface of the carrier 35 before it escapes from the developing area. This is considered to cause white spots in the halftone image adjacent to the front end of the solid image.

It has been found that the following seven parameters are involved in white spots in the halftone image portion adjacent to the solid image. (A) Gap Gp between developer carrier and image carrier (hereinafter referred to as “development gap”), (B) Developer amount Dq per unit area on developer carrier, (C) Image carrier Ratio of linear velocity Vs / Vp of developer carrier surface to body surface, (D) AC voltage frequency f [kHz], (E) toner weight concentration TC [wt%], (G) unit for obtaining maximum image density 7 parameters of toner adhesion amount per area (M / A) [mg / cm ² ].

(A) When the developing gap Gp is narrowed, the above-described edge effect is reduced, and halftone whiteout (all-out whiteout) in a solid image adjacent portion is reduced. This is because if the development gap is small, the wraparound of the electric field is reduced. Further, if the development gap is narrowed, it is easy to obtain a high image density.
However, if the development gap is too narrow, the amount of developer (pumping amount) supplied to the development area must be reduced, and the amount of toner that can be used for development will be reduced. There is a need.
(B) When the developer amount Dq per unit area on the developer carrying member is high, the movement of the carrier in the development nip is suppressed, and the carrier from which the solid image is developed and the toner is peeled off forever. May remain at the tip of the magnetic brush. Such a carrier at the tip of the magnetic brush receives a counter charge, and the toner is peeled off from the halftone image portion around the solid image, thereby causing halftone voids in the adjacent portion of the solid image. On the other hand, if the developer amount Dq per unit area on the developer carrier is small, there is a risk of leakage from the developer carrier to the image carrier. Note that the amount of developer per unit area on the developer carrying member is measured by the amount conveyed to the development region.
(C) When the linear velocity ratio Vs / Vp of the surface of the developer carrying body to the surface of the image carrying body is reduced, halftone white spots in the solid image adjacent portion are reduced.
With respect to “all-round white spots”, it is improved by increasing the linear velocity ratio Vs / Vp of the developer carrier surface to the image carrier surface. This is presumably because the edge effect becomes weaker as the linear velocity ratio Vs / Vp of the developer carrier surface to the image carrier surface is increased. On the other hand, it has been experimentally found that with respect to “blank tip”, it is improved by reducing the linear velocity ratio Vs / Vp of the surface of the developer carrier to the surface of the image carrier. There are two contradictory mechanisms for the linear velocity ratio Vs / Vp of the developer carrier surface to the image carrier surface and white spots, but as a whole, the line of the developer carrier surface with respect to the image carrier surface. It has been found that reducing the speed ratio Vs / Vp is more advantageous for suppressing halftone white spots in the solid image adjacent portion.
However, if the linear velocity ratio Vs / Vp of the surface of the developer bearing member to the surface of the image bearing member is too small, the developing ability may be lowered. Accordingly, it is necessary not to make the linear velocity ratio Vs / Vp of the surface of the developer carrier to the surface of the image carrier too small.
(D) When the frequency f of the alternating voltage is increased, charge injection into the image carrier is suppressed and background stains are reduced. However, if the frequency is too high, an edge effect is likely to occur, and halftone white spots in a solid image adjacent portion are likely to occur.
(E) When the toner weight concentration TC is reduced, the counter charge per unit area is reduced. Therefore, white spots in the halftone image portion adjacent to the solid image are reduced. However, if the value is too low, the developing ability is deteriorated.
(F) It was found that when the peak-to-peak voltage [kV] of the AC voltage is reduced, halftone white spots in the solid image peripheral portion tend to decrease. This is considered because the edge effect became weak.
The reason why an AC voltage is applied to the developer carrying member and an AC bias is applied to the developing nip is to improve the developing ability. When an AC bias is applied to the development nip, the toner carried on the magnetic brush is easily moved, and the toner attached to the carrier near the base of the magnetic brush is also easily transferred to the image carrier side by the development bias.
(G) The toner adhesion amount (M / A) per unit area for obtaining the maximum image density is a toner adhesion amount necessary for obtaining a target image density ID. This value is greatly related to the ratio of the coloring component in the toner component (hereinafter referred to as “coloring degree”). That is, the toner adhesion amount (M / A) per unit area for obtaining the maximum image density is reduced in the toner with a high degree of coloring, and the toner per unit area for obtaining the maximum image density in the toner with a low degree of coloring. The toner adhesion amount (M / A) increases.

  The present invention was completed by earnestly researching how the above seven parameters affect each other.

(1) The present inventor multiplies the toner weight concentration TC [unit: wt%] by an AC voltage peak-to-peak voltage value Vpp [unit: kV] for applying an AC developing bias to the developer carrier. Correlation between TC × Vpp / (M / A) obtained by dividing this value by the adhesion amount per unit area (M / A) [unit: mg / cm ² ] for obtaining the maximum image density and the blank width I found that there is a relationship.
An example is shown in FIG. In the figure, TC × Vpp / (M / A) is plotted against the blank area. The experimental conditions at this time are as shown in Tables 1 and 2. As a device, Ricoh's imgio color 5105 it machine was remodeled so as to satisfy desired setting conditions. This apparatus employs a multi-value writing method as a writing method.
Here, as described above, white spots in the halftone image portion adjacent to the solid image include “all-round white spots” and “front end white spots”. Therefore, the white spots appear most conspicuously in the halftone image portion adjacent to the front end portion of the solid image where white spots are overlapped in both (see FIG. 5). If the whiteout width is larger than 2 [mm], it will be noticeable on the fixed image. Therefore, the present inventor decided to set the blank width of the halftone image portion adjacent to the solid image front end portion to be 2 [mm] or less.

6 and Table 3, if the condition (1) TC × Vpp / (M / A) ≦ 14 is satisfied, the blank width of the halftone image portion adjacent to the solid image front end portion is set to 2.0 [mm. It can be seen that the following can be achieved.

Further, among the seven parameters described above, the remaining four parameters not related to the condition (1) satisfy the above condition (1) and suppress white spots in the halftone image adjacent to the solid image. We studied repeatedly to find a possible range. As a result, it has been found that if the above conditions (2) to (5) are satisfied, the blank width at the leading edge of the solid image can be set to 2.0 [mm] or less.
In addition, a toner having a higher degree of coloring than a commonly used toner (that is, a toner having a small amount of adhesion (M / A) per unit area for obtaining the maximum image density (for example, 0.40 of Experimental Example 1). [Mg / cm ² ])) was also examined, and it was found that as long as the above conditions (1) to (5) were satisfied, white spots in the halftone image adjacent to the solid image could be prevented.

(2) The present inventor multiplies the development gap Gp [unit: mm] by the developer amount Dq [unit: mg / cm ² ] per unit area on the developer carrying member, and uses this value as the maximum image density. It was found that there is a correlation between Gp × Dq / (M / A) divided by the adhesion amount per unit area (M / A) [unit: mg / cm ² ] and the whiteout width. .
An example is shown in FIG. In the figure, Gp × Dq / (M / A) is plotted against the white space. The experimental conditions at this time are as shown in Tables 4 and 5. The other conditions are the same as in (1) above.

7 and Table 6, if Gp × Dq / (M / A) ≦ 60, which is the above condition (6), is satisfied, the blank width of the halftone image portion adjacent to the front end portion of the solid image is 2.0 [mm. It can be seen that the following can be achieved.

Further, among the seven parameters described above, the remaining four parameters not related to the condition (6) satisfy the above condition (6) and suppress white spots in the halftone image adjacent to the solid image. We studied repeatedly to find a possible range. As a result, it has been found that if the above conditions (7) to (10) are satisfied, the blank width at the leading edge of the solid image can be set to 2.0 [mm] or less.
In addition, a toner having a higher degree of coloring than a commonly used toner (that is, a toner having a small amount of adhesion (M / A) per unit area for obtaining the maximum image density (for example, 0.40 in Experimental Example 14). [Mg / cm ² ])) was also examined, and it was found that as long as the above conditions (6) to (10) were satisfied, white spots in the halftone image adjacent to the solid image could be prevented.

(3) The ratio of the linear velocity Vs / Vp of the developer carrier surface to the image carrier surface is divided by the frequency f [unit: kHz] of the AC voltage, and this value is further obtained per unit area for obtaining the maximum image density. It was found that there is a correlation between (Vs / Vp) / f / (M / A) divided by the adhesion amount (M / A) [unit: mg / cm ² ] and the whiteout width.
An example is shown in FIG. In the figure, (Vs / Vp) / f / (M / A) is plotted against the void width. The experimental conditions at this time are as shown in Tables 7 and 8. Other conditions are the same as in (1) above.

8 and Table 9, if the condition (11) (Vs / Vp) / f / (M / A) ≦ 0.75 is satisfied, the blank width of the halftone image portion adjacent to the solid image front end portion. It can be seen that can be made 2.0 [mm] or less.

Further, among the seven parameters described above, the remaining four parameters not related to the condition (11) satisfy the above condition (11) and suppress white spots in the halftone image adjacent to the solid image. We studied repeatedly to find a possible range. As a result, it has been found that if the above conditions (12) to (15) are satisfied, the blank width at the leading edge of the solid image can be set to 2.0 [mm] or less.
In addition, a toner having a higher degree of coloring than a commonly used toner (that is, a toner having a small amount of adhesion (M / A) per unit area for obtaining the maximum image density) (for example, 0.4 in Experiment 27) [Mg / cm ² ])) was also examined, and it was found that as long as the above conditions (6) to (10) were satisfied, white spots in the halftone image portion adjacent to the solid image could be prevented.

Embodiment 1
Hereinafter, an embodiment (hereinafter, this embodiment is referred to as “Embodiment 1”) in which a color printer (hereinafter simply referred to as “printer”) as an image forming apparatus is applied to the present invention will be described.
FIG. 9 is a schematic configuration diagram of the printer according to the first embodiment. In the drawing, around a photosensitive member 1 on a drum as an image carrier, there are a charging charger 2 as a charging means, a photosensitive member potential detecting device 3, an exposure device 4, a rotary developing device 5, and a reflection density. A sensor 6, an intermediate transfer member 8, a photosensitive member cleaning device 7, and the like are disposed. The intermediate transfer member 8 rotates in the direction of the arrow in the figure, a primary transfer charger 9 is disposed at a position facing the photoconductor 1, and a secondary transfer charger 11 and an intermediate transfer member cleaning device 10 are disposed. Yes.

  The rotary developing device 5 includes four developing devices 19Bk, 19C, 19M, and 19Y. Each developing device develops Bk (black), Y (yellow), M (magenta), and C (cyan). The developing sleeves 16Bk, 16C, 16M, and 16Y of each developing device are rotated so that the developing sleeves 16Bk, 16C, 16M, and 16Y are located at the developing position with the central axis of the rotary developing device 5 as the center of rotation. It is configured to perform development.

An image forming process in this image forming apparatus will be described. First, the first color Bk is developed. At this time, the rotary developing device 5 is in a position where the Bk developing device 19Bk faces the photosensitive member 1. The surface of the photosensitive member 1 that is rotationally driven in the direction of the arrow is uniformly charged from the charging charger 2. A latent image corresponding to the image of the document is formed on the surface of the photosensitive member 1 by the exposure device 4 based on an image signal from an image processing unit (not shown). The latent image on the photosensitive member 1 is developed with a developer 33Bk (see FIG. 10) supplied from the developing sleeve 16Bk of the developing device 19Bk at a portion facing the rotary developing device 5, and becomes a visible image. The visible image is transferred onto the intermediate transfer member 8 by the primary transfer charger 9. Here, the untransferred toner on the photoconductor 1 is cleaned from the photoconductor 1 by the photoconductor cleaning device 7. When the development of Bk is completed, the rotary developing device 5 rotates 90 degrees and stops at a position where the next developing device 19C faces the photoreceptor 1.
Thereafter, the same operation as described above is repeated, the remaining developing devices 19C, 19M, and 19Y are developed, and the developed images are transferred onto the intermediate transfer member 8 in a superimposed manner. The toner overlaps.

  Subsequently, the four color visible images on the intermediate transfer member 8 are fed at a predetermined timing from a sheet feeding device (not shown) and conveyed to a portion facing the intermediate transfer member 8, for example, transfer paper 15 as a recording member. Are transferred by the secondary transfer charger 11. The transfer sheet 15 on which the visible image is transferred is transported to the fixing devices 13 and 14 by the transport device 12, and the visible image is fixed and discharged. Further, the untransferred toner on the intermediate transfer member 8 after the visible image transfer is cleaned from the intermediate transfer member 8 by the intermediate transfer member cleaning device 10.

  The rotary developing device 5 includes four developing devices 19Bk, 19C, 19M, and 19Y, and toner supply devices 30Bk, 30C, 30M, and 30Y that supply toners 36Bk, 31C, 31M, and 31Y to the developing devices. Has been. The rotary developing device 5 is driven by a drive motor (not shown). Therefore, when the rotary developing device 5 is rotated, the four developing devices and the four toner supply devices rotate together.

FIG. 10 is an explanatory diagram of a developing device used in the printer having the above configuration. Since each developing device 19Bk, 19C, 19M, 19Y has the same configuration, only the developing device 19Bk for Bk will be described.
The Bk developing device 19Bk of the image forming apparatus performs development using a two-component developer 33Bk composed of toner and carrier. The developing device 19Bk includes a casing having an opening toward the photosensitive member 1, a developing sleeve 16Bk as a developer carrying member for supplying the developer 33Bk onto the photosensitive member 1 from the opening, and development in the developing device 19Bk. The first stirring roller 17Bk, the second stirring roller 18Bk, and the like as developer stirring means for stirring the agent 33Bk.
The toner supply device 30Bk includes a toner cartridge that accommodates the toner 36Bk to be supplied to the developing device 19Bk, a toner supply screw 32Bk as supply means for supplying the toner in the toner cartridge into the developing device, and the like. Yes.
The toner 36Bk in the toner cartridge of the toner supply device 30Bk is supplied into the developing device by the toner supply screw 32Bk. In the developing device, the stirring rollers 17Bk and 18Bk replenish the developer consisting of toner and carrier in the casing onto the developing sleeve 16Bk while mixing and stirring the developer. The developer replenished on the developing sleeve 16Bk is regulated to a predetermined thickness, and then conveyed to a developing area opposite to the photosensitive member 1 to develop the latent image on the photosensitive member 1.

  The toner weight concentration TC [wt%] of the developer in the developing case is detected by a toner concentration sensor (not shown), and the toner is replenished from the toner supply device 30Bk so as to constantly become a desired toner weight concentration during the image forming operation. It is configured to be.

The surface movement direction of the developing sleeve 16 in the developing region is a direction along with the photosensitive member 1, and the linear velocity is set faster than the linear velocity of the photosensitive member 1. In this case, the magnetic brush moves relative to the electrostatic latent image so as to rub while overtaking the electrostatic latent image on the photoreceptor 1. That is, the surface of the photoconductor is rubbed so as to be sequentially overtaken by a plurality of magnetic brushes while passing through the development region.
A vibration bias voltage in which an AC voltage is superimposed on a DC voltage is applied to the developing sleeve as a developing bias voltage by a power source (not shown). The background portion potential and the image portion potential are located between the maximum value and the minimum value of the vibration bias potential. As a result, an alternating electric field whose direction changes alternately is formed at the developing position where the photosensitive member 1 and the developing sleeve 16 face each other. During this alternating electrolysis, the toner and carrier of the developer vibrate vigorously, and the toner flies off the photosensitive sleeve 1 by shaking off the electrostatic binding force to the developing sleeve 16 and the carrier, and corresponds to the latent image on the photosensitive body 1. Adhere to.
Note that the DC component of the AC voltage is set so as to obtain an adhesion amount (M / A) [mg / cm ² ] per unit area for obtaining a desired image density.

Next, the characteristic part of the first embodiment will be described.
In the first embodiment, the following conditions (1) to (5) are set.
Condition (1): TC × Vpp / (M / A) ≦ 14 (where Vpp is a peak-to-peak voltage value [unit: kV] for applying an AC developing bias to the developing sleeve, and TC is a toner. (Weight density [unit: wt%], (M / A) is the adhesion amount [unit: mg / cm ² ] per unit area for obtaining the maximum image density.)
Condition (2): The closest distance Gp between the developing sleeve and the photosensitive member is 0.30 [mm] or more and 0.60 [mm] or less. Condition (3): The developer amount Dq per unit area on the developing sleeve is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less Condition (4): The linear velocity ratio Vs / Vp of the developing sleeve surface to the photoreceptor surface is 1.2 or more and 2.0 or less (5 ): The frequency f of the AC developing bias applied to the developing sleeve is 2.0 [kHz] or more and 8.0 [kHz] or less. The toner weight concentration TC is obtained by a generally used toner weight concentration measuring device. be able to. However, it is necessary to select a mesh that is generally used for measurement and through which the carrier does not pass.

By setting in this way, white spots in the halftone image portion of the solid image adjacent portion can be suppressed and a high-quality image can be provided. The reason is as described with reference to FIG.
However, the following problems may occur depending on the type of developer. Specifically, when the value of the toner weight concentration TC is less than 4 [wt%], there is a possibility that the carrier adheres to the photoreceptor. On the other hand, if it is larger than 8 [wt%], the problem of background contamination may occur. Further, if the peak-to-peak voltage Vpp of the AC voltage is less than 0.4 [kV], the developing ability may be insufficient. On the other hand, if it is larger than 0.8 [kV], there is a possibility that an image defect due to a discharge phenomenon occurs in the developing nip portion.
Therefore, it is more desirable that the value of the toner weight concentration TC is 4 [wt%] or more and 8 [wt%] or less. The peak-to-peak voltage Vpp of the AC voltage is more preferably 0.4 [kV] or more and 0.8 [kV] or less.
The lower limit value of TC × Vpp / (M / A) ≦ 14, which is the condition (1), is preferably set to be 6 or more in consideration of the parameter setting range.

As described above, it is desirable to reduce the adhesion amount (M / A) per unit area for obtaining the maximum image density from the viewpoint of resource saving. This value is largely related to the degree of coloration of the toner. That is, the toner adhesion amount (M / A) per unit area for obtaining the maximum image density is reduced in the toner with a high degree of coloring, and the toner per unit area for obtaining the maximum image density in the toner with a low degree of coloring. The toner adhesion amount (M / A) increases. From the viewpoint of resource saving, the value of the amount of adhesion (M / A) per unit area for obtaining the maximum image density is desirably 0.40 [mg / cm ² ] or less. In order to set in this way, it is necessary to set the ratio of the degree of coloring in the toner higher than in the conventional case. This value is preferably as small as possible, but it is appropriate to set it to 0.2 [mg / cm ² ] or more in the current technology in consideration of image quality and the like.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
[Example 1]
Under the experimental conditions shown in Table 10, the halftone blank width of the solid image adjacent portion was evaluated. As a printer, Ricoh's imgio color 5105 it machine was remodeled as necessary so as to satisfy the desired conditions. This apparatus employs a multi-value writing system.

[Example 2]
Except for the conditions shown in Table 11, the halftone blank width of the solid image adjacent portion was evaluated under the same conditions as in Example 1 above.

[Comparative Example 1]
Except for the conditions shown in Table 12, the halftone blank width of the solid image adjacent portion was evaluated under the same conditions as in Example 1 above.

The evaluation results for the above examples and comparative examples are summarized in Table 13 below. The allowable levels for each evaluation item are as follows.
[Halftone blank in the solid image adjacent area]
The image output was evaluated by measuring the whiteout width.

  From Table 13, by satisfying the conditions (1) to (5), it is possible to suppress white spots in the halftone image adjacent to the solid image and obtain a high-quality image even in the multi-level writing method. all right. Further, even when the degree of coloring is higher than that in the past (that is, when the amount of adhesion per unit area for obtaining the maximum image density is small (Example 2)), white spots in the halftone image adjacent to the solid image are eliminated. It turned out that it can suppress.

[Embodiment 2]
Next, as in the first embodiment, another embodiment in which the present invention is applied to a printer as an image forming apparatus (hereinafter, this embodiment is referred to as “second embodiment”) will be described.
The basic configuration and operation of the apparatus are the same as those in the first embodiment, but the parameter setting method is different.
In the second embodiment, the following conditions (6) to (10) are satisfied.
Condition (6): Gp × Dq / (M / A) ≦ 60 (where Gp is the closest distance [unit: mm] between the developing sleeve and the photosensitive member, and Dq is per unit area on the developing sleeve) Developer amount [unit: mg / cm ² ].)
Condition (7): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (8): The peak-to-peak voltage value Vpp of the AC voltage for applying the AC developing bias to the developing sleeve is 0 .4 [kV] or more and 1.0 [kV] or less Condition (9): The linear velocity ratio Vs / Vp of the developing sleeve surface to the photosensitive member is 1.2 or more and 2.0 or less. Condition (10): Developing sleeve The frequency f of the AC developing bias applied to the voltage is 2.0 [kHz] or more and 8.0 [kHz] or less

By doing so, white spots in the halftone image portion of the solid image adjacent portion can be suppressed, and a high-quality image can be provided. The reason is as described with reference to FIG.
However, the following problems may occur depending on the type of developer. Specifically, if the value of the development gap is too small, it is not preferable because the crossing width needs to be narrowed and the mechanical structure becomes complicated. On the other hand, if the development gap is too large, a problem of toner scattering may occur. Further, if the amount of the developer per unit area on the developing sleeve is too small, the developing ability is lowered, and there is a fear that a trace (magnetic brush mark) may appear on the solid portion of the halftone. On the other hand, if the amount of the developer per unit area on the developer sleeve is too large, the developer may be fixed in the development region and an abnormal image may be generated, which is not preferable.
Therefore, the development gap is more preferably set to 0.25 [mm] or more and 0.45 [mm] or less. The developer amount per unit area on the developer sleeve is more preferably 30 [mg / cm ² ] or more and 60 [mg / cm ² ] or less.
Further, the lower limit value of Gp × Dq / (M / A) ≦ 60, which is the condition (6), is preferably set to 25 or more in consideration of the setting range of the parameters.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
[Example 3]
Under the conditions shown in Table 14, the halftone white spots in the solid image adjacent portion were evaluated. The printer used is the same as in the first embodiment.

[Example 4]
Except for the conditions shown in Table 15, the halftone blank width of the solid image adjacent portion was evaluated under the same conditions as in Example 3 above.

[Comparative Example 2]
Except for the conditions shown in Table 16, the halftone blank width at the solid image adjacent portion was evaluated under the same conditions as in Example 3 above.

Table 17 summarizes the evaluation results for the above examples and comparative examples.

  From Table 17, by satisfying the conditions (6) to (10), even in the multi-value writing method, white spots in the halftone image adjacent to the solid image are suppressed, and a high-quality image can be obtained. all right. Further, even when the degree of coloring is higher than in the past (that is, when the amount of adhesion per unit area for obtaining the maximum image density is small (Example 4)), white spots in the halftone image adjacent to the solid image are eliminated. It was found that high-quality images can be obtained with suppression.

[Embodiment 3]
Next, as in the first embodiment, another embodiment in which the present invention is applied to a printer as an image forming apparatus (hereinafter, this embodiment is referred to as “second embodiment”) will be described.
The basic configuration and operation of the apparatus are the same as in the first embodiment, but the parameter setting method is different.
In the third embodiment, the following conditions (11) to (15) are satisfied.
Condition (11): (Vs / Vp) / f / (M / A) ≦ 0.75 (where Vs / Vp is the developing sleeve surface linear velocity / photoreceptor surface linear velocity, and f is applied to the developing sleeve. AC development bias frequency [unit: kHz].)
Condition (12): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (13): The peak-to-peak voltage value Vpp of the AC voltage for applying the AC developing bias to the developing sleeve is 0 .4 [kV] or more and 1.0 [kV] or less Condition (14): The closest distance Gp between the developing sleeve and the photosensitive member is 0.30 [mm] or more and 0.60 [mm] or less. Condition (15) : Developer amount Dq per unit area on the developing sleeve is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less

By doing so, white spots in the halftone image portion of the solid image adjacent portion can be suppressed, and a high-quality image can be provided. The reason is as described with reference to FIG.
However, the following problems may occur depending on the type of developer. Specifically, if the developing sleeve surface linear velocity / photoreceptor surface linear velocity is less than 1.1, there is a possibility that the developing ability may be insufficient, or scumming may occur. On the other hand, if it exceeds 1.6, the life of the developer may be shortened. Further, if the frequency f of the AC developing bias is less than 4.0 [kHz], there is a risk that background staining will occur. On the other hand, if the frequency f of the AC developing bias is made higher than 12.0 [kHz], the edge effect may be likely to occur.
Therefore, the developing sleeve surface linear velocity / photosensitive member surface linear velocity is preferably 1.1 or more and 1.6 or less. The frequency f of the AC developing bias is preferably 4.0 [kHz] or more and 12.0 [kHz] or less.
Further, the lower limit value of (Vs / Vp) / f / (M / A) ≦ 0.75, which is the condition (11), is preferably set to 0.35 or more in consideration of the setting range of the parameters.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
[Example 5]
Under the conditions shown in Table 18, the halftone blank width of the solid image adjacent portion was evaluated. The printer used is the same as in the first embodiment.

[Example 6]
Except for the conditions shown in Table 19, the halftone blank width of the solid image adjacent portion was evaluated under the same conditions as in Example 5 above.

[Comparative Example 3]
Except for the conditions shown in Table 20, the halftone blank width at the solid image adjacent portion was evaluated under the same conditions as in Example 5 above.

Table 21 summarizes the evaluation results for the above examples and comparative examples. The allowable level in each evaluation item is as described above.

  From Table 21, by satisfying the conditions (11) to (15), it is possible to suppress white spots in the halftone image adjacent to the solid image and obtain a high-quality image even in the multi-level writing method. all right. Further, even when the degree of coloring is higher than that of the conventional case (that is, the amount of adhesion per unit area for obtaining the maximum image density is small (Example 6)), white spots in the halftone image adjacent to the solid image are eliminated. It was found that high-quality images can be obtained with suppression.

  The printer used in the above embodiment is an example of an apparatus to which the present invention can be applied, and is not limited to this apparatus. Further, the setting conditions of each device and member are not limited to those of the present embodiment.

The figure for demonstrating the image formation method of a binary writing system and a multi-value writing system. The figure for demonstrating the mechanism in which an all-around white spot generate | occur | produces. The figure for demonstrating the mechanism in which an all-around white spot generate | occur | produces. The figure for demonstrating the mechanism in which a tip white spot generate | occur | produces. The figure for demonstrating the blank of the halftone image adjacent to a solid image. The figure which plotted TC * Vpp / (M / A) with respect to the white-out width. The figure which plotted GpxDq / (M / A) with respect to the white-out width. The figure which plotted (Vs / Vp) / f / (M / A) with respect to the blank area width. 1 is a schematic configuration diagram of a printer according to an embodiment. FIG. 2 is a schematic configuration diagram of a developing device of the present embodiment. 1 is a schematic configuration diagram of a conventional developing device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor 5 Developing apparatus 16 Developing sleeve 33 Developer 35 Carrier 36 Toner

Claims (11)

A developing device having a developer carrying member carrying a two-component developer comprising a toner and a carrier, and a developing bias applying means for applying a developing bias containing an AC component to the developer carrying member;
In an image forming apparatus provided with an image carrier that forms an electrostatic latent image facing the developer carrier,
An image forming apparatus satisfying the following conditions (1) to (5):
Condition (1): TC × Vpp / (M / A) ≦ 14 (where Vpp is a peak-to-peak voltage value [unit: kV] of an AC voltage for applying an AC developing bias to the developer carrier. TC is the toner weight concentration [unit: wt%], and (M / A) is the toner adhesion amount [unit: mg / cm ² ] per unit area for obtaining the maximum image density.)
Condition (2): The closest distance Gp between the developer carrier and the image carrier is 0.30 [mm] or more and 0.60 [mm] or less. Condition (3): Unit on the developer carrier Developer amount Dq per area is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less Condition (4): The linear velocity ratio Vs / Vp of the surface of the developer carrier to the image carrier is 1.2 or more and 2.0 or less Condition (5): The frequency f of the AC developing bias applied to the developer carrying member is 2.0 [kHz] or more and 8.0 [kHz] or less. The image forming apparatus according to claim 1.
An image forming apparatus, wherein a peak-to-peak voltage value Vpp of an AC voltage for applying an AC developing bias to the developer carrying member is set to 0.4 [kV] or more and 0.8 [kV] or less. The image forming apparatus according to claim 1 or 2,
An image forming apparatus, wherein the toner weight concentration TC is 4 [wt%] or more and 8 [wt%] or less. A developing device having a developer carrying member carrying a two-component developer comprising a toner and a carrier, and a developing bias applying means for applying a developing bias containing an AC component to the developer carrying member;
In an image forming apparatus provided with an image carrier that forms an electrostatic latent image facing the developer carrier,
An image forming apparatus satisfying the following conditions (6) to (10):
Condition (6): Gp × Dq / (M / A) ≦ 60 (where Gp is the closest distance [unit: mm] between the developer carrier and the image carrier, and Dq is the developer carrier) The amount of developer per unit area [unit: mg / cm ² ].)
Condition (7): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (8): Peak-to-peak voltage value of AC voltage for applying AC developing bias to the developer carrying member Vpp is 0.4 [kV] or more and 1.0 [kV] or less. Condition (9): The linear velocity ratio Vs / Vp of the surface of the developer carrier to the image carrier is 1.2 or more and 2.0. Condition (10): The frequency f of the AC developing bias applied to the developer carrier is 2.0 [kHz] or more and 8.0 [kHz] or less. The image forming apparatus according to claim 4.
An image forming apparatus, wherein a closest distance Gp between the developer carrier and the image carrier is 0.25 [mm] or more and 0.45 [mm] or less. The image forming apparatus according to claim 4 or 5,
An image forming apparatus, wherein a developer amount Dq per unit area on the developer carrying member is 30 [mg / cm ² ] or more and 60 [mg / cm ² ] or less. A developing device having a developer carrying member carrying a two-component developer comprising a toner and a carrier, and a developing bias applying means for applying a developing bias containing an AC component to the developer carrying member;
In an image forming apparatus provided with an image carrier that forms an electrostatic latent image facing the developer carrier,
An image forming apparatus satisfying the following conditions (11) to (15):
Condition (11): (Vs / Vp) / f / (M / A) ≦ 0.75 (where Vs / Vp is the linear velocity ratio of the developer carrier surface to the image carrier surface, and f is (It is the frequency [unit: kHz] of the AC developing bias applied to the developer carrying member.)
Condition (12): Toner weight concentration TC is 6 [wt%] or more and 9 [wt%] or less Condition (13): Peak-to-peak voltage value of AC voltage for applying AC developing bias to the developer carrying member Vpp is 0.4 [kV] or more and 1.0 [kV] or less Condition (14): The closest distance Gp between the developer carrier and the image carrier is 0.30 [mm] or more and 0.60 [mm] ] The following condition (15): The developer amount Dq per unit area on the developer carrier is 40 [mg / cm ² ] or more and 70 [mg / cm ² ] or less. The image forming apparatus according to claim 7.
An image forming apparatus, wherein a value of a linear velocity ratio Vs / Vp of the developer carrying member surface to the image carrying member surface is 1.1 or more and 1.6 or less.   9. The image forming apparatus according to claim 7, wherein the frequency f of the AC developing bias applied to the developer carrier applied to the developer carrier is 4.0 [kHz] or more and 12.0 [kHz] or less. An image forming apparatus. The image forming apparatus according to claim 1, 2, 3, 4, 5, 6, 7, 8, or 9.
An image forming apparatus characterized by adopting a multi-value writing system as a system for forming an electrostatic latent image on the image carrier. The image forming apparatus according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.
An image forming apparatus, wherein a toner adhesion amount (M / A) per unit area for obtaining the maximum image density is 0.40 [mg / cm ² ] or less.